FIELD OF THE INVENTION
The invention relates to a method for adapting mechanical tolerances when performing time measurements on rotating shafts, in particular on internal combustion engine crankshafts or shafts connected thereto.
One possible way of measuring time is by providing the shaft itself, or preferably a sensor wheel connected thereto, with marks and scanning the marks through the use of a sensor. The time within which the shaft rotates through a specific angle of rotation or through a specific segment is measured. In that way, fluctuations in rotational speed are sensed to a very high resolution, in addition to the rotational speed, as a function of the distance between the marks on the periphery of the shaft or the sensor wheel.
Such measurements are used, for example, in internal combustion engines, in order to detect combustion misfires by reference to a brief slowing down of the angular speed of the crankshaft. However, if the marks or the sensor wheel have mechanical faults or inaccuracies, such as, for example, angular faults of the segments or faults in marking teeth on the sensor wheel, eccentricity of the disk or deviations in the tooth shape, the measurement of the angular speed is falsified. As a consequence thereof, misdiagnoses may be made when detecting combustion misfires.
The determination of adaptation values for correcting such faults is described, for example, in European Patent EP 0 583 495 B1 which is assigned to the assignee of the instant application.
In that method, the segment times are measured for a reference segment at an interval of two crankshaft revolutions. In that way it is possible to compensate for a general change in the tendency of the rotational speed, which would otherwise lead to faulty corrections. The further segment times which are measured between those two measurements are compared with the segment time of the reference segment, and a correction value, which makes it possible to correct the measured segment time on a cylinder-specific basis, is determined as a function of the calculated time difference.
Under certain circumstances, it is possible for maladaptations to occur with the known methods according to the prior art, for example in internal combustion engines which are equipped with a two-mass flywheel on the crankshaft. In addition to relieving the crankshaft of unacceptable bending stresses, such a two-mass flywheel brings about an improvement in the acoustics and the sensation of comfort. The two-mass flywheel may be embodied as a spring-mass system, and thus have a natural frequency which is dependent, inter alia, on the spring stiffness, on the two masses and on the coefficients of friction. At critical rotational speeds of the internal combustion engine (for example 2500 rpm), a superimposition of oscillations may then occur, in particular if an oscillation is superimposed on the 1.5 order of engine. That oscillation disrupts the adaptation algorithm and leads to faulty adaptation of the measured time values, as a result of which combustion misfires are incorrectly detected.